As one of the methods for writing address information on an optical disk, and as a method which can meet the demand for a high density optical disk, a method in which address information is written on a tracking control-use groove or land is available. In this method, the tracking control-use groove or land is wobbled, and a component of a wobbling frequency is extracted from a tracking error signal so as to find address information.
Japanese Unexamined Patent publication No. 314538/1993 (Tokukaihei 5-314538) discloses such a method. FIG. 10(a) through FIG. 10(c) are explanatory drawings explaining the method. As shown in FIG. 10(a) through FIG. 10(c), an optical disk adopting this method is provided with a groove 50 on a disk substrate 52 (substrate part). The groove 50 is provided such that only one of the side walls is wobbled in accordance with the address information. The address information is found by projection of a recording-reproducing-use spot 51 so as to read only one side of the groove 50. The recording-reproducing spot 51 is smaller than two times the width of the groove So.
However, in order to manufacture such a disk substrate 52, as shown in FIG. 10(b), it is required to project at least two light beams 53a and 53b which are separated from one another in a radius direction of the optical disk, and during the projection, only one of the light beams, the light beam 53a, is wobbled in the radius direction in accordance with the address information. That is to say, when manufacturing the disk substrate 52, the light beams 53a and 53b are projected so as to follow the alternate long and short lines 54 and 55, respectively, of FIG. 10(b). This presents problems that (1) the efficiency of light utilization is lowered due to the separation of a light beam into two beams, (2) the optical system becomes complex, and (3) the light beams 53a and 53b are required to be controlled independently.
In order to solve such problems, inventors of the present invention have proposed a disk substrate in Japanese Patent Application No. 176199/1996 (Tokuganhei 8-176199). Such a disk substrate has an arrangement wherein a tracking control-use tracking guides composed of grooves or lands are provided with (1) a wobbling part which is wobbled in accordance with address information and (2) a non-wobbling part. The wobbling part and the non-wobbling part are provided alternately in a radius direction the optical disk.
In the disk substrate having the described arrangement, only a single light beam is required for manufacturing thereof, thereby increasing the efficiency of the light utilization, and simplifying the optical system. Also, even in the case of reducing the size of a track pitch on the disk substrate in order to record information on the optical disk in high density, it is possible to accurately read the address information.
Incidentally, in Japanese Patent Application No. 176199/1996 (Tokuganhei 8-176199), as a method for reproducing an optical disk provided with the described disk substrate, the following method is proposed. According to this method, address information is reproduced from the wobbling part of the tracking guides by a single light beam for recording and reproducing data information. However, in the case where the address information is to be reproduced by a single light beam, it is possible that the light beam deviates off the center of the track (region where information is recorded) when the optical disk is tilted or vibrated. Hence, in such an event, address information cannot be reproduced by the proposed reproducing method.
Specifically, as shown in FIG. 11(a), with a single light beam, even when reproducing is carried out in a normal condition, a side end portion of a light beam 57 is projected on tracking guides composed of a wobbling groove 56a which is wobbled and a normal groove 56b which is not wobbled. Thus, when the light beam 57 deviates off the center of a track 58 (dotted lines in FIG. 11(a) and FIG. 11(b)), which is a region between the wobbling groove 56a and the normal groove 56b, in response to slight tilting of the optical disk or other causes, as shown in FIG. 11(b), the wobbling groove 56a and the light beam 57 deviate from one another with ease.
In such an event, the tracking error signal is obtained only from the left side portion of the light beam 57 such that the light beam 57 is returned to the center of the track 58 by controlling of a tracking servo system. However, while the controlling is carried out, the address information cannot be found from the wobbling groove. This presents a problem that the reproducing of address information becomes unstable.
Also, in the case of carrying out reproducing with a single light beam 57 in the described manner, the wobbling groove 56a is always under the side end portion of the light beam 57. This presents a drawback that the amount of signals of the address information thus obtained is small.